SKY REPORTER: April 2012

Sky Data

Sunday April 1st, sunrise is at 6:39 a.m. Eastern Daylight Time. That evening the sun, located in the constellation Pisces, sets at 7:21 p.m.

On Sunday April 15th sunrise is at 6:16 a.m., and sunset occurs that day at 7:36 p.m.

The last day of the month, Monday April 30th, the sun is in Aries, rising at 5:55 a.m. and setting at 7:51 that evening. During April the length of time the sun is above the horizon each day increases by one hour 14 minutes. The altitude of the sun at noon, its maximum daily elevation, ascends during April from 54 degrees of arc on the first to 64 degrees on the 30th.

During this month average overnight temperatures recorded in Central Park near Belvedere Castle rise from 37 degrees Fahrenheit on the first to 46 degrees on the 30th.

Principal Phases of the Moon are:

April 6

Full Moon

April 13

Last Quarter

April 21

New Moon

April 29

First Quarter

Sky Cast

On Sunday April 1st, Mercury rises in the predawn sky at 5:55 a.m., just 33 minutes before the sun. The following evening Venus is a magnificent sight in the western sky, above bright Jupiter. Mars is also above the horizon at the end of civil twilight, high in the southeast. Saturn rises at 8:22 p.m. That night the waxing gibbous moon is in between Mars and the bright zero magnitude star Procyon in Canis Minor.

During evenings from April first through Wednesday the 4th, Venus passes the tiny dipper shaped asterism in the midst of the Pleiades open star cluster in Taurus. Binoculars or a low power telescope will show these beautiful objects as Venus nightly moves eastward through the zodiac. Its passages relative to “fixed stars” such as the Pleiades reinforce our understanding of why planets were known as “wandering stars.” During April Venus’ crescent phase narrows and appears to lengthen as the planet’s distance from Earth diminishes preceding its much anticipated June conjunction and transit of the Sun’s disk.

Monday and Tuesday April 2nd and 3rd, the waxing gibbous moon may be seen eight degrees from Regulus, brightest star in the constellation Leo.

By the night of Friday April 6th the moon will have moved to a position just west of Saturn and first magnitude star Spica in Virgo.

During the early morning of Tuesday April 10th the waning gibbous moon is four degrees north of first magnitude red supergiant Antares in Scorpius. The next night the moon is not far from the direction of the supermassive black hole 26,000 light years from us at the center of the Milky Way Galaxy.

April 14th Saturn is in opposition, rising at sunset and remaining in the sky through the night.

Wednesday April 18th Mercury is at its greatest western elongation, in the early morning sky 27 degrees of arc west of the sun measured along the ecliptic. However it is only three degrees above the eastern horizon when civil twilight begins on the morning of the 18th. Locating Mercury this month is as usual a difficult task.

During evening twilight on April 22nd a thin crescent moon is low in the western sky, three degrees above Jupiter. On the evenings of the 24th and 25th, the moon is within 12 degrees of Venus.

Open star cluster M44, also known as the Beehive or Praesepe cluster.

Credit: NOAO/AURA/NSF

The night of Saturday April 28th the moon is seven degrees of arc southwest of the Beehive open star cluster in the constellation of the Crab. The cluster, also known as the Praesepe (Latin for “manger”) and as Messier 44, extends across an area of the sky about four times that covered by the moon’s disk. At a distance of about 800 light years this represents an actual diameter of 20 light years. Although visible to unaided eyes on very clear dark nights, looking like a “little mist”, binoculars easily show this cluster. The Beehive is estimated to be 580 light years from the Solar System, with an age of about 800 million years. Look a half fist length above the moon on April 28th to view the celestial Beehive.

Sky Lore

During April the Big Dipper and Leo anchor center stage in the early evening sky, with the Dipper high in the north near the zenith. Although having fainter stars than many we saw on winter evenings, the Big Dipper joins Regulus in Leo, Arcturus in Bootes, and Spica in Virgo, marking special places in our celestial view this season. Seven Big Dipper stars comprise a significant part of Ursa Major, the Great Bear and it is an unmistakable pattern. Its delicate esthetic is remarkable and unforgettable.

The Big Dipper contains the fine stellar pair Mizar and Alcor, marking a bend in the asterism’s handle. They and four other Dipper stars belong to the Ursa Major moving cluster, traveling together through the Milky Way Galaxy. Known in sky lore as the “horse and rider” Mizar and Alcor are separated by 12 arcminutes, about one-third the diameter of the moon’s disk. The respective distances from Earth of Mizar and Alcor are 78 and 81 light years. Observers with excellent vision may see Alcor, at visual magnitude 4, and Mizar at magnitude 2.2 as separate points of light. If vision is not quite 20/20, binoculars show the pair’s separation. Focusing our attention on Mizar, using a telescope with magnification of at least 50 times, this star can be seen as double, its components separated by 14 arcseconds (equivalent to about 500 times the distance from Earth to the Sun). Although an orbital connection between Alcor and Mizar may exist, components of Mizar are definitely linked by gravity and revolve around their common center of mass over a period of more than 5,000 years. Furthermore, each is a very close double star, with separations revealed by spectroscopic analysis.

When you have located the Big Dipper, use it as a guide to other features of April’s sky. For example, with a bit of imagination we can think of the Dipper’s bowl filled with water that suddenly spills down onto Leo, the celestial Lion. Regulus is Leo’s brightest star and it is 77 light years from us. It is the first magnitude star closest to the ecliptic, the sun’s apparent annual path through our sky. Regulus means “little king”, a name said to have been provided by Copernicus.

Referring again to the Big Dipper, we can trace a curve along its handle to the star Alkaid. Extend an imagined arc about 30 degrees (three fist lengths) to the zero magnitude star Arcturus in the constellation Bootes, the herdsman. In our night sky Arcturus is second only in stellar brilliance to Sirius and is a highlight of springtime nights. Arcturus is 37 light years from us and has a slightly orange color. It has a relatively fast southward motion quite different from other stars in the neighborhood of the Sun and is described as a “high velocity” star. Arcturus may have been a member of a small galaxy that collided with and was absorbed by the Milky Way Galaxy about five billion years ago.

Continuing the imaginary curve southward past Arcturus brings our attention to first magnitude Spica, brightest star in Virgo. It and Arcturus were known in Arabian mythology as representing posts supporting the canopy of the sky.